Determining the degree to which chloroplast genome copy number limits the expression of chloroplast genes

Presenter: Dylan Udy, Biology

The relationship that exists between chloroplasts and the plant cells they occupy is very complex. Chloroplasts evolved from a cyanobacterial endosymbiont, and throughout evolution many of the ancestral bacterial genes have been transferred to the plant nuclear genome. The proteins from many such nuclear genes are sent back to the chloroplast where they perform a variety of functions. We identified a non-photosynthetic maize mutant that accumulates reduced levels of several chloroplast mRNAs. We showed that the causal mutation is a transposon insertion in a nuclear gene encoding a protein that is closely-related to bacterial DNA polymerase I. Angiosperm genomes include two closely related paralogs encoding this protein. These have been studied in Arabidopsis (a dicot plant), where they are dual-targeted to both the mitochondria and chloroplast and have redundant functions. I have shown that our maize mutant has a 10-fold reduction in chloroplast DNA but normal levels of mitochondrial DNA, suggesting that the two paralogs have become specialized for either chloroplast or mitochondrial DNA replication in maize (a monocot plant). I am using the maize mutant to investigate the degree to which the abundance of chloroplast DNA limits chloroplast gene expression. I have found that the abundance of some chloroplast mRNAs decreases in parallel with the abundance of chloroplast DNA, whereas the abundance of other mRNAs does not. These results show that different factors limit the expression of different genes in the chloroplast.